³²S/³⁴S isotope ratios as functions of the distance from the Galactic center. The symbol ⊙ indicates the ³²S/³⁴S isotope ratio in the Solar System. In the upper panel, the ³²S/³⁴S ratios directly derived from ¹³CS/¹³C³⁴S in the J = 2−1 transition and obtained from the double isotope method in the J = 2−1 transition with corrections for optical depth are plotted as black and green dots, respectively. The ³²S/³⁴S ratios without corrections of opacity in the J = 3−2 transition are plotted as light blue dots. The 3σ lower limits of ³²S/³⁴S ratios obtained from non-detections of ¹³C³⁴S in the current work are shown as open black circles. The ³²S/³⁴S ratios in Yu et al. (2020) derived from the double isotope method in the J = 2−1 transitions are shown as blue dots in the lower panel. The ³²S/³⁴S values in the CMZ obtained from ¹³CS/¹³C³⁴S in Humire et al. (2020) are plotted as red stars in both panels. The resulting first-order polynomial fits to ³²S/³⁴S ratios with the direct method and the double isotope method from the J = 2−1 transition in this work are plotted as black and red solid lines in the two panels, respectively, with the gray and yellow shaded areas showing the 1 σ intervals of the fits. The magenta and cyan dashed-dotted lines show the ³²S/³⁴S gradients from Chin et al. (1996) and Yu et al. (2020). The red crosses visualize the results from the GCE model of Kobayashi et al. (2011, 2020, see also Sect. 4.9).